Local area networks utilize a variety of collision detection techniques to provide efficient and potentially trouble-free media access. The collision detection techniques are selected in accordance with particular local area network attributes such as the type of signal encoding (FSK, OOK, PSK or the like), the type of multiple access scheme (TDMA, CSMA or the like), and the configuration of the network (bus, ring, star, etc.).
Collision detection techniques used with Manchester-encoded, carrier sense multiple access, local area networks, such as the STARLAN NETWORK, are separated into three distinct categories. The first collision detection technique searches for Manchester code violations in the data signal on the transmission medium in either a daisy chain or star configuration. A second collision detection technique recognizes two or more station nodes transmitting at the same time from a star configuration. Yet another collision detection technique involves the recognition of a lost carrier signal in either a daisy chain or star configuration.
Manchester encoded signals display the unique property that within each and every bit period there exists a transition from one state (high or low) to the opposite state (low or high, respectively). Interference from other signals can cause the Manchester encoded signal to exhibit a smeared, jittered or obliterated mid-bit transition. Such behavior at the mid-bit transition is known as a Manchester violation. Manchester violations are detected in either a daisy chain or star configuration by differential detection circuits and other similar techniques known in the art.
In the star configuration, a hub, which is centrally connected to each station node, detects energy signals from the nodes. Upon sensing signaling activity by two or more station nodes at the same time, the hub generates a signal indicative of the presense of a signal collision. The signal generated by the hub effectively jams all communication in the local area network and causes Manchester code violations to appear on the transmission medium. Station nodes in the star configuration detect this jamming signal and interpret the Manchester code violations as collisions.
Lost carrier collision detection is implemented at the station nodes, i.e., end points, of the local area network. When a station node transmits data and does not see its own carrier, that is, its own transmitted signal, returning after a predetermined time period, the station node assumes that a collision has occurred.
In a daisy chain configuration, the lost carrier collision detection technique theoretically detects collisions when two stations in the daisy chain begin simultaneous transmission with antipodal data signals. However, in practice, this technique even when augmented with a Manchester code violation collision detector fails to detect more than an insignificant number of collisions.